Differences between gravitational and electromagnetic radiation

So far we have been emphasizing how, at a fundamental level, the
generation and propagation of gravitational and electromagnetic
radiation are basically quite similar. This is a major point in
demystifying gravitational waves. But, on a more practical level,
gravitational and electromagnetic waves are quite different: we see
and use electromagnetic waves every day, while we have yet to make a
confirmed direct detection of gravitational waves (which is why they
seemed so mysterious in the first place).

There are two principal differences between gravity and
electromagnetism, each with its own set of consequences for the nature
and information content of its radiation, as described below.

Gravity is a weak force, but has only one sign of charge.
Electromagnetism is much stronger, but comes in two opposing
signs of charge.

This is the most significant difference between gravity and
electromagnetism, and is the main reason why we perceive these
two phenomena so differently. It has several immediate
consequences:

Significant gravitational fields are generated by
accumulating bulk concentrations of matter. Electromagnetic
fields are generated by slight imbalances caused by small
(often microscopic) separations of charge.

Gravitational waves, similarly, are generated by the bulk
motion of large masses, and will have wavelengths much
longer than the objects themselves. Electromagnetic waves,
meanwhile, are typically generated by small movements of
charge pairs within objects, and have wavelengths
much smaller than the objects themselves.

Gravitational waves are weakly interacting, making them
extraordinarily difficult to detect; at the same time, they
can travel unhindered through intervening matter of any
density or composition. Electromagnetic waves are strongly
interacting with normal matter, making them easy to detect;
but they are readily absorbed or scattered by intervening
matter.

Gravitational waves give holistic,
sound-like information about the overall motions
and vibrations of objects. Electromagnetic waves give
images representing the aggregate properties of
microscopic charges at the surfaces of objects.

Gravitational charge is equivalent to inertia.
Electromagnetic charge is unrelated to inertia.

This is the more fundamental difference between
electromagnetism and gravity, and influences many of the details
of gravitational radiation, but in itself is not responsible for
the dramatic differences in how we perceive these two types of
radiation. Most of the consequences of the principle of
equivalence in gravity have already be discussed, such as:

The fundamental field of gravity is a gravitational
force gradient (or tidal) field, and
requires an apparatus spread out over some distance in order
to detect it. The fundamental field in electromagnetism is
an electric force field, which can be felt by
individual charges within an apparatus.

The dominant mode of gravitational radiation is
quadrupolar: it has a quadratic dependence on the
positions of the generating charges, and causes a relative
"shearing" of the positions of receiving charges. The
dominant mode of electromagnetic radiation is
dipolar: it has a linear dependence on the
positions of the generating charges, and creates a relative
translation of the positions of receiving charges.